Lawsuit says social network should have made security issues clearer.

The social media giant did not provide users with enough warning that their Facebook accounts could be compromised and their data stolen due to security flaws in its sign-on system, a new lawsuit has said.

Facebook is being sued in the US following last year's attack, which left around 29 million user accounts open to hackers.

Facebook security risk

The accusation came as part of an ongoing court case lodged by a group of Facebook users following the September 2018 breach. Hackers were able to acccess millions of Facebook accounts after stealing "access tokens" used to log in users connecting via third-party apps and services.

Users would sign in to the site via these services using their Facebook login details, meaning that gaining control of the access tokens would give attackers entry to full accounts.

“Facebook knew about the access token vulnerability and failed to fix it for years, despite that knowledge,” the court filing at the U.S. District Court for the Northern District of California in San Francisco read.

“Even more egregiously, Facebook took steps to protect its own employees from the security risk, but not the vast majority of its users.”

Of those affected in the attack, around 14 million saw profile details such as birth dates, employers, education history, religious preference, types of devices used, pages followed and recent searches and location check-ins stolen.

Names and contact details were exposed for the other 15 million users, with around 400,000 others having their posts and lists of friends and groups open.

Facebook has yet to respond to the filing, but faces yet more questioning over its security protection systems.

The company Facebook was fined a record $5bn by the US Federal Trade Commission (FTC) last month to settle privacy concerns over the Cambridge Analytica data-stealing scandal.